The invention relates to apparatus for a specimen grip assembly which securely holds a test specimen in a jaw assembly of a materials testing machine. The present invention is well suited for use in a variety of different mechanical and thermo-mechanical materials testing machines, including those which provide compressive and tensile testing of a mechanical test specimen.
2. Description of the Prior Art
Conventional materials testing machines hold an end of a test specimen in proper position in a jaw during a mechanical test through any one of various mechanisms, including specimen grips, collets, or fixtures.
In such a machine, typically a known and controlled compressive and/or tensile force is applied, often under computer control and as defined by a so-called “test program”, to one of two specimen grips that collectively hold opposite ends of the specimen and thus deforms the specimen under predefined test conditions. Each grip is securely held in a corresponding jaw assembly, with one such assembly being moveable while the other is fixed in position. Often, a test program may encompass applying a series of such forces—tensile and/or compressive, i.e., commonly referred to “hits”, to the moveable jaw and therethrough to the specimen to increasingly deform the specimen. Dilation and/or other physical measurements are typically made of the amount of resulting deformation as or after each hit is made to the specimen. In a thermo-mechanical materials testing system, the specimen may also be controllably and self-resistively heated, such as through serial passage of electrical heating current through both the jaws and specimen, before, simultaneously with or after each such hit and as defined in the test program. Such systems are exemplified by the GLEEBLE dynamic thermo-mechanical materials testing systems presently manufactured by Dynamic Systems, Inc. of Troy, N.Y., which is the present assignee (GLEEBLE being a registered trademark of Dynamic Systems, Inc.).
One rather common method of holding a test specimen in such a materials testing machine is through use of a pair of wedge-shaped specimen grips which collectively surrounds each end of the specimen and securely fits into an appropriately shaped jaw assembly. The jaw assembly has two opposing inclined faces where each such face abuttingly and slideably mates with a complementary shaped face in a corresponding one of the pair of grips. The specimen is positioned between both grips. Thereafter, but prior to the commencement of a test program, the two specimen grips are jacked into position such that their inclined faces increasingly abut and slideably move against both faces of the jaw assembly and, through wedge-action of these mating faces, both force the grips into a highly fixed and secure position within the jaw assembly and also force the specimen grips toward each other to effectively lock the specimen in its position.
An externally generated force, typically through, e.g., a rod of a servo-controlled piston connected to the moveable jaw, is applied to that particular jaw to produce each such “hit”.
Unfortunately, several drawbacks arise in a materials testing system through use of such a conventional wedge-based specimen grip and jaw assembly.
First, since the force used to deform the specimen is applied to the specimen through the moveable jaw assembly, the specimen grips and the jacking mechanism, the specimen grips must be capable of withstanding peak stresses considerably far in excess of those applied to the test specimen with minimal resulting distortion, and moreover repeatedly so during a cyclic application of such force during a test program. Some distortion, referred to as “compliance”, of the jaw assembly invariably occurs. The amount of compliance that occurs is governed by the amount of force to which the jaw assembly is subjected as well as the design itself of the assembly, including its geometry and material. Disadvantageously, compliance of the jaw assembly adversely injects error into the test results for a simple reason that the compliance reduces an amount of deformation that would otherwise occur in the specimen itself resulting from each hit. Ideally, a jaw assembly, under full load, would have no compliance whatsoever, however this is generally not possible in an actual materials testing machine.
A conventional way to reduce compliance is to use relatively large and massive jaws. However, massive jaws adversely influence not only measurement of the force, but also, as more force is required to move such jaws than otherwise, reduce acceleration of, e.g., a piston rod that is used to generate the force for each hit and thus reduce its stroke rate. Reductions in the stroke rate consequently reduce a rate of deformation that can be produced in the specimen. This, in turn, concomitantly lessens an ability of the materials testing system to accurately simulate actual material working conditions that occur in, for example, certain high-speed metal production operations, such as a high-speed rolling mill, and thus, in practice, lowers the attractiveness to use such systems in such simulations. To minimize these adverse effects, the jaw assemblies should be as light-weight as possible so as not to interfere with both force measurements and acceleration produced by the piston and the ensuing rate of specimen deformation produced. However, doing so has unfortunately tended to increase compliance, thus precluding use of light-weight jaws.
Second, a conventional jaw typically has a recess which accommodates the specimen grips. Whenever a tensile force is externally applied to the jaws, the jaw exhibits compliance. Specifically as a result of the tensile force, the recess, particularly at its top edges, will tend to spread apart and distort somewhat thus diminishing the force which would otherwise hold the specimen grips together. Similarly, the vertical surfaces of the specimen grips, that would otherwise secure a proximate portion of the end of the specimen, will also exhibit compliance and tend to separate apart. This causes a loss of specimen alignment and side loading of the specimen which, in turn, may too adversely affect the accuracy of the test results. Here again, to appropriately reduce this compliance, the jaw assembly can be made of a relatively large, heavy and massive material. But, as noted above, increasing the mass of the jaw assembly adversely limits the stroke rate and hence effectively limits machine performance in accurately simulating high-speed metal production operations.
Consequently, a need exists in the art for a specimen grip assembly, preferably employing wedge-shaped specimen grips, for use in a jaw assembly of a materials testing machine, that not only exhibits substantially little, if any, noticeable compliance during the application of tensile and/or compressive forces during a succession of “hits” but also can be used in a relatively light-weight jaw.